All joints should be planned and selected based on the amount, and direction of force that will be applied to the joint. Any joint that lack it's own mechanical integerity and relies on an adhesive alone will fail, on it's own, in time.
The endgrain to facegrain butt joint is the second weakest joint there is. End grain to end grain butt being the weakest. It relies on the strength of the adhesive property, or the mechaincal strength of the fastener used.
A dado provides a few mechanical functions;
It provides for precise alignment.
It provides shear load strength. ( resistance to force applied 90 degrees to the joint)
It provides axial load strength.(resistance to twisting forces)
The dado by itself is very poor for forces being applied in line with the joint. If you build a freestanding shelf with no backing, or individual shelf supports. Using only dados and rabbets. You have created a reflex parallelogram.
Clamp the structure to a bench and push on the top corner and you will break it very easily. Glued and screwed will make it stronger, but the small amount of leverage applied will break even these joints.
I personally would not use pocket screws alone. The strength is based on the properties of the screw, and the type of wood it's being screwed into. ( I wouldn't use drywall screws at all. They are made from a very brittle material and shear a lot easier than you might think.)
For this project (the shelves, top, and bottom at least). I would dado the shelves, and bottom, rabbett the top, and screw and glue. I would screw from the face side into the dado joint.
A pocket hole jig would provide less that 3/4" of screw engagment in the applicaion of attaching the shelves, top, and bottom to the sides. A screw gets it's mechanical strength from it's length. I'd be using 8-32 , or 10-32 screws,2" long, screwed into the joint.
Build it stout, you won't regret it.
In this project,
http://www.sawmillcreek.org/showthread.php?t=63386 Every joint is a dado joint, that is screwed and glued (epoxied).
The screws were screwed into the dado's from the face side, and wenge plugs cover the screwheads.
The resistance to "racking" forces is provided by the back, which is a combination of individually rabbeted, shiplapped, and dado'd boards.
My vote is Dado, glue, and screw.
Use the pocket hole jig for the face frames.